Late to this thread... aren't those upsized from stock 19s? (225 and 255 respectively).

Not much of a tire expert (read not at all).... what does this do to speedo/odo and/or other things?

That tire size at the rear (265/30 vs. 255/30) has a slight increased rear tire diameter and will actually improve speedometer error from the typical BMW +3 mph over actual to about +1 over actual. Stock size with 225/255 setup has a bit more propensity to understeer due to 30mm tread width difference from front to rear. Less understeer is accomplished by closing that 30mm gap to 20mm with a 245/265 setup. Even more neutral handling with a Stage 3 Dinan setup (their rims) with a 265/30 + 275/30 setup, with spot-on accurate speedometer.